Driver circuitry

ABSTRACT

Driver circuitry for driving an electromechanical load with a drive output signal, the driver circuitry comprising: a first control loop operable to control the drive output signal based on a drive input signal; and a second control loop operable to control the drive output signal based on a current flowing through and/or a voltage induced across the electromechanical load, wherein the second control loop is configured to have a lower latency than the first control loop, and to control the drive output signal to compensate for an impedance of the electromechanical load.

The present disclosure is a continuation of U.S. patent application Ser.No. 16/928,106, filed Jul. 14, 2020, which is a continuation of U.S.patent application Ser. No. 16/369,556, filed Mar. 29, 2019, issued asU.S. Pat. No. 10,828,672 on Nov. 10, 2020, each of which is incorporatedby reference herein in its entirety.

FIELD OF DISCLOSURE

The present disclosure relates in general to driver circuitry, inparticular for use in driving an electromechanical load or device. Oneexample of an electromechanical load (electromechanical device) is anactuator such as a linear resonant actuator (LRA).

The present disclosure extends to methods carried out by such drivercircuitry and to systems, such as haptic systems, comprising such drivercircuitry.

BACKGROUND

Driver circuitry may be implemented (at least partly on ICs) within ahost device (host apparatus), which may be considered an electrical orelectronic device and may be a mobile device. Example host devicesinclude a portable and/or battery powered host device such as a mobiletelephone, a smartphone, an audio player, a video player, a PDA, amobile computing platform such as a laptop computer or tablet and/or agames device.

As is well known, haptic technology recreates the sense of touch byapplying forces, vibrations, or motions to a user. Haptic devices(devices enabled with haptic technology) may incorporate tactile sensors(input transducers) that measure forces exerted by the user on a userinterface (such as a button or touchscreen on a mobile telephone ortablet computer) as well as output transducers (electromechanical loads)which apply forces directly or indirectly (e.g. via a touchscreen) to auser. Taking a haptic system as an example, with an LRA serving as theelectromechanical load, driver circuitry may be employed to drive theLRA to produce a haptic effect (such as a vibration or other tactilesensation) for a user. Audio-to-haptic conversion may also be employedfor example in connection with a user playing a video game, to convertan audio signal into a corresponding haptic signal to provide a tactilesensation (output via an electromechanical load such as an LRA)alongside an audio signal (output via a speaker).

The main components of an LRA are a voice coil, a moveable magneticmass, a spring and a casing or chassis. The magnetic mass is connectedto the spring which in turn is mounted to the casing or chassis of theLRA. An AC voltage signal (a drive signal) is used to drive the voicecoil, which is arranged to magnetically couple with the moveablemagnetic mass.

An LRA typically produces an oscillating force or vibration along anaxis. When the voice coil is driven with the AC voltage signal(particularly at the resonant frequency of spring-mass arrangement), theresultant magnetic field induces movement in the magnetic mass andcauses it to vibrate with a human-perceptible force. It is the vibrationof the mass with a perceptible force which provides the haptic effect.Essentially, the frequency and amplitude of the AC voltage signal isconverted into a vibrational frequency and amplitude of the magneticmass connected to the spring. The LRA is thus a form of transducer. LRAsare typically highly resonant, and as such are generally driven at theirresonant frequency for efficiency, i.e. to optimise the relationshipbetween the haptic effect and power consumption.

Of course, an LRA is one example type of electromechanical load (beingan actuator or transducer), which is particularly suitable for producinga haptic effect for a user in the context of host devices as mentionedabove. Driver circuitry may be used to drive other types ofelectromechanical load (electromechanical device), e.g. which can bemodelled as a resonant actuator such as a speaker or microspeaker orwhich have non-resonant mechanical loads such as a solenoid or voicecoil motor that is non-resonant.

The example of driving an LRA will be carried forward herein in thecontext of haptic systems as a convenient running example.

Accuracy of control of actuators and transducers is important, forexample in the field of haptic technology (e.g. haptic feedback). Thequality of the user haptic experience where an LRA is used is defined bythe accuracy of control of the LRA, for example.

It is desirable to provide improved driver circuitry with this in mind,to improve control (e.g. mechanical control) of an electromechanicalload driven by that circuitry.

SUMMARY

According to a first aspect of the present disclosure, there is provideddriver circuitry for driving an electromechanical load with a driveoutput signal based on a digital reference signal at a first samplerate, the drive output signal inducing a first electrical quantity atthe electromechanical load. The driver circuitry may comprise: afunction block configured, based on said first electrical quantity, todigitally determine at a second sample rate higher than the first samplerate an adjustment signal indicative of a second electrical quantitywhich would be induced at a target output impedance of the drivercircuitry due to said first electrical quantity; and a driver configuredto generate the drive output signal based on the reference signal andthe adjustment signal to cause the drive output signal to behave as ifan output impedance of the driver circuitry has been adjusted tocomprise the target output impedance. The first electrical quantity maybe a current and the second electrical quantity may be a voltage, orvice versa.

By digitally determining the adjustment signal, it is possible to causethe drive output signal to behave as if the output impedance of thedriver circuitry has been adjusted to comprise the target outputimpedance in a highly adaptable and controllable manner. Further, bydigitally determining the adjustment signal at the second sample ratehigher than the first sample rate, the target output impedance isimplemented over a relatively wide bandwidth.

The drive output signal may be a voltage signal (voltage mode control).In that case, the first electrical quantity may be a current drawn bythe electromechanical load and the second electrical quantity may be avoltage across the target output impedance.

The drive output signal may be a current signal (current mode control).In that case, the first electrical quantity may be a voltage across theelectromechanical load and the second electrical quantity may be acurrent drawn by the target output impedance.

The function block may be configured to digitally determine theadjustment signal based on the first electrical quantity and adefinition of the target output impedance. For example, the definitionmay comprise one or more configuration values. The driver circuitry maycomprise (or have access to) storage for storing the one or moreconfiguration values, wherein an impedance value of the target outputimpedance is maintained when the one or more configuration values storedin the storage are maintained. That is, the impedance value of thetarget output impedance may be dependent on the configuration values.

A (hypothetical) target equivalent circuit representative of the targetoutput impedance may comprise one or more impedance components and acircuit structure for connecting the one or more impedance componentstogether. The one or more configuration values may define at least onesaid impedance component and/or said circuit structure.

The target equivalent circuit may comprise a plurality of impedancecomponents connected together. The function block may be configured,based on said first electrical quantity and the one or moreconfiguration values, to: determine a plurality of adjustmentsub-signals each representative of a corresponding part of the targetequivalent circuit and indicative of a portion of the second electricalquantity which would be induced at the corresponding part of the targetequivalent circuit if said second electrical quantity were induced atthe target equivalent circuit; and determine the adjustment signal bycombining the plurality of adjustment sub-signals.

The portion of the second electrical quantity may be a voltage acrossthe corresponding part of the target equivalent circuit if the secondelectrical quantity is the voltage across the target output impedance.The portion of the second electrical quantity may be a current drawn bythe corresponding part of the target equivalent circuit if the secondelectrical quantity is the current drawn by the target output impedance.

The one or more configuration values may define the target equivalentcircuit to comprise at least one of a series resistor, a seriescapacitor, a series inductor and a parallel network of impedances. Theparallel network of impedances may comprise at least two of a parallelresistor, a parallel capacitor and a parallel inductor connectedtogether in parallel. Each of those resistors, capacitors and inductorsmay be considered a said impedance component.

Those of the series resistor, the series capacitor, the series inductorand the parallel network of impedances present in the target equivalentcircuit may be connected in series, for example where the secondelectrical quantity is the voltage across the target output impedance.

The one or more configuration values may define the target equivalentcircuit to comprise, optionally only, the series resistor, wherein theseries resistor has a negative resistance (for example substantiallyequal in magnitude to the positive resistance of a voice coil of theelectromechanical load).

The one or more configuration values may define the target equivalentcircuit to comprise, optionally only, the series resistor and the seriesinductor connected together in series, wherein the series resistor has anegative resistance (for example substantially equal in magnitude to thepositive resistance of a voice coil of the electromechanical load) andthe series inductor has a negative inductance (for example substantiallyequal in magnitude to the positive inductance of a voice coil of theelectromechanical load).

The one or more configuration values may define the target equivalentcircuit to comprise, optionally only, the series resistor and the seriesinductor connected together in series and to the parallel network ofimpedances, wherein the series resistor has a negative resistance andthe series inductor has a negative inductance, and wherein the parallelnetwork of impedances comprises the parallel resistor, the parallelcapacitor and the parallel inductor connected together in parallel.

The one or more configuration values may define the target equivalentcircuit to comprise, optionally only, the series resistor and the seriescapacitor connected together in series, wherein the series resistor hasa negative resistance and the series capacitor has a positivecapacitance.

The one or more configuration values may define the target equivalentcircuit to comprise, optionally only, the series resistor, wherein theseries resistor has a positive resistance, and wherein the positiveresistance is substantially larger than a resistance of theelectromechanical load, or than a resistance of a resistor in anelectromechanical-load equivalent circuit representing a mechanicalimpedance of the electromechanical load.

The driver circuitry may comprise a controller. The controller may beconfigured to generate the reference signal based on a drive inputsignal and based on a current drawn by the electromechanical load and/ora voltage across the electromechanical load. The controller may beconfigured to control, based on said current drawn by theelectromechanical load and/or said voltage across the electromechanicalload, a definition of the target output impedance to cause a performancesuch as a mechanical performance of the electromechanical load to meet aperformance target. The controller may be configured to control, basedon said current drawn by the electromechanical load and/or said voltageacross the electromechanical load, a definition of the target outputimpedance to cause the target output impedance to cancel an impedance ofat least one electrical component of the electromechanical load,optionally a coil such as a voice coil. The controller may be configuredto control a definition of the target output impedance based on animpedance control signal to cause a performance of the driver circuitryto vary with the impedance control signal.

The driver may be configured to generate the drive output signal so thatthe drive output signal has a predefined relationship with a summationof the adjustment signal and the reference signal.

The function block may be configured to generate a control signal havinga predefined relationship with a summation of the adjustment signal andthe reference signal. The driver may be configured to generate the driveoutput signal so that the drive output signal has a predefinedrelationship with the control signal.

The driver circuitry may be selectively operable in an impedance-drivemode or a current-drive mode. When the driver circuitry is in theimpedance-drive mode, the control signal is generated based on thereference signal and the adjustment signal so that the drive outputsignal behaves as if the output impedance of the driver circuitry hasbeen adjusted to comprise the target output impedance (as mentionedearlier). In the current-drive mode, the function block may beconfigured to generate the control signal as a function of acurrent-control reference signal and a current drawn by theelectromechanical load, and to adjust the control signal based on saidcurrent drawn by the electromechanical load so that said current drawnby the electromechanical load has a predefined relationship with thecurrent-control reference signal.

At least one of the control signal and the adjustment signal may be adigital signal. The control signal and the adjustment signal may bedigital signals, and the function block may be a digital function block(e.g. implemented in digital hardware, or in software running on aprocessor). The drive output signal may be referred to as an analoguesignal.

The control signal may be a digital signal. The driver may comprise adigital-to-analogue converter and an analogue amplifier connectedtogether to convert the control signal into an analogue signal and thenamplify that analogue signal to form the drive output signal.

The driver circuitry may comprise a monitoring unit configured togenerate a current monitoring signal indicative of a current drawn bythe electromechanical load and/or a voltage monitoring signal indicativeof a voltage across the electromechanical load. The function block maybe configured to digitally determine the adjustment signal based on thecurrent monitoring signal and/or the voltage monitoring signal.

The reference signal may be indicative of an intended mechanicalperformance of the electromechanical load. The behaviour of the driveoutput signal as if the output impedance of the driver circuitry hasbeen adjusted to comprise the target output impedance may be relative toan expected behaviour of an expected drive output signal expected to begenerated by the driver based on the reference signal without theadjustment signal (or based on the adjustment signal having a zerovalue). The driver circuitry may comprise one or more analogue impedancecomponents connected to contribute to the output impedance of the drivercircuitry. The target output impedance may be configured to cancel animpedance of at least one electrical component of the electromechanicalload, optionally a coil such as a voice coil. The electromechanical loadmay be an electromechanical device such as an actuator. Theelectromechanical load may be a resonant electromechanical load such asa linear resonant actuator, a speaker or a microspeaker.

It may be considered that the driver forms part of a first control loopoperable to control the drive output signal based on the referencesignal. The driver and the function block may be considered to form partof a second control loop operable to control the drive output signalbased on a current drawn by the electromechanical load and/or a voltageacross the electromechanical load. The second control loop may beconfigured to have a lower latency than the first control loop.

At least part of the first control loop and at least part of the secondcontrol loop may be implemented as digital circuitry. The latencies ofthe first and second control loops may be defined by sample rates ofrespective digital signals of the first and second control loops.

The driver circuitry may comprise an analogue impedance configured toform part of the output impedance of the driver circuitry. The analogueimpedance may be a controllable analogue impedance and the functionblock may be configured to control the controllable analogue impedanceto adjust the output impedance of the driver circuitry. For example, thedriver circuitry may be configured to control a definition of the targetoutput impedance and/or an impedance of the analogue impedance tocontrol the output impedance of the driver circuitry.

The driver circuitry may be implemented as integrated circuitry such ason an IC chip.

According to a second aspect of the present disclosure, there isprovided an IC chip comprising the driver circuitry according to theaforementioned first aspect of the present disclosure.

According to a third aspect of the present disclosure, there is provideda control system, comprising: the driver circuitry according to theaforementioned first aspect of the present disclosure; and theelectromechanical load, wherein the electromechanical load is connectedto be driven by said drive output signal.

According to a fourth aspect of the present disclosure, there isprovided a haptic system comprising the control system of theaforementioned third aspect of the present disclosure, wherein theelectromechanical load is a linear resonant actuator (or other type ofactuator) coupled to a physical structure or surface of the system toproduce a haptic effect for a user.

According to a fifth aspect of the present disclosure, there is provideda host device, such as portable electrical or electronic device,comprising the driver circuitry according to the aforementioned firstaspect of the present disclosure, or the IC chip of the aforementionedsecond aspect of the present disclosure, or the control system of theaforementioned third aspect of the present disclosure or the hapticsystem of the aforementioned fourth aspect of the present disclosure.

According to a sixth aspect of the present disclosure, there is provideda method carried out by driver circuitry to drive an electromechanicalload with a drive output signal based on a digital reference signal, thedrive output signal inducing a first electrical quantity at theelectromechanical load, the method comprising: based on said firstelectrical quantity, digitally determining at a second sample ratehigher than the first sample rate an adjustment signal indicative of asecond electrical quantity which would be induced at a target outputimpedance of the driver circuitry due to said first electrical quantity;and generating the drive output signal based on the reference signal andthe adjustment signal to cause the drive output signal to behave as ifan output impedance of the driver circuitry has been adjusted tocomprise the target output impedance, wherein the first electricalquantity is a current and the second electrical quantity is a voltage,or vice versa.

According to a seventh aspect of the present disclosure, there isprovided driver circuitry for driving an electromechanical load with adrive output signal, the driver circuitry comprising: a first controlloop operable to control the drive output signal based on a drive inputsignal; and a second control loop operable to control the drive outputsignal based on a current flowing through and/or a voltage inducedacross the electromechanical load, wherein the second control loop isconfigured to have a lower latency than the first control loop.

The second control loop may be configured to control the drive outputsignal to compensate for an impedance of the electromechanical load. Thesecond control loop may be configured to control the drive output signalso that it behaves as if an output impedance of the driver circuitry hasbeen adjusted to comprise a target output impedance.

The drive output signal may be a voltage signal and the second controlloop may be configured to perform its control of the drive output signalbased on a voltage signal which would be induced across the targetoutput impedance by said current. The second control loop may beconfigured to determine, based on said current, an adjustment signalindicative of said voltage signal, and control the drive output signalbased on the adjustment signal.

The drive output signal may be a current signal and the second controlloop may be configured to perform its control of the drive output signalbased on a current signal of a current which would be induced to flowthrough the target output impedance by said voltage. The second controlloop may be configured to determine, based on said voltage, anadjustment signal indicative of said current signal, and control thedrive output signal based on the adjustment signal.

According to an eighth aspect of the present disclosure, there isprovided driver circuitry for driving an electromechanical load with adrive output signal based on a reference signal, the drive output signalinducing a first electrical quantity at the electromechanical load, thedriver circuitry comprising: a function block configured, based on saidfirst electrical quantity, to digitally determine an adjustment signalindicative of a second electrical quantity which would be induced at atarget output impedance of the driver circuitry due to said firstelectrical quantity; and a driver configured to generate the driveoutput signal based on the reference signal and the adjustment signal tocause the drive output signal to behave as if an output impedance of thedriver circuitry has been adjusted to comprise the target outputimpedance.

The drive output signal may be a voltage signal. In that case, the firstelectrical quantity may be a current drawn by the electromechanical loadand the second electrical quantity may be a voltage across the targetoutput impedance.

The drive output signal may be a current signal. In that case, the firstelectrical quantity may be a voltage across the electromechanical loadand the second electrical quantity may be a current drawn by the targetoutput impedance.

According to a ninth aspect of the present disclosure, there is provideddriver circuitry for driving an electromechanical load with a driveoutput signal based on a reference signal, the drive output signal beinga voltage signal and causing a current to be drawn by theelectromechanical load, the driver circuitry comprising: a functionblock configured, based on said current, to digitally determine anadjustment signal indicative of a voltage signal which would be inducedacross a target output impedance of the driver circuitry by saidcurrent; and a driver configured to generate the drive output signalbased on the reference signal and the adjustment signal to cause thedrive output signal to behave as if an output impedance of the drivercircuitry has been adjusted to comprise the target output impedance.

According to a tenth aspect of the present disclosure, there is provideddriver circuitry for driving a linear resonant actuator, the drivercircuitry comprising: a function block configured to generate a digitalcontrol signal as a function of a digital reference signal, intended forcontrolling the linear resonant actuator, and a monitor signal; and adriver configured to convert the digital control signal into an analoguedrive signal to drive the linear resonant actuator, wherein: the monitorsignal is indicative of a current flowing through, and/or a voltageacross, the linear resonant actuator; and the function block isconfigured, based on the monitor signal, to control a difference betweenthe digital control signal and the digital reference signal so that theanalogue drive signal when driving the linear resonant actuator has atarget behaviour in which the analogue drive signal behaves, relative toan expected analogue drive signal expected to be generated with thedigital control signal being the digital reference signal, as if theoutput impedance of the driver circuitry has been adjusted to comprise atarget output impedance.

According to an eleventh aspect of the present disclosure, there isprovided driver circuitry for driving an electromechanical load with adrive output signal based on a reference signal, the driver circuitryconfigured to generate the drive output signal based on a digitaloperation dependent on the reference signal and an electrical quantityinduced at the electromechanical load to cause the drive output signalto behave as if an output impedance of the driver circuitry has beenadjusted to comprise a target output impedance.

According to a twelfth aspect of the present disclosure, there isprovided driver circuitry for driving an electromechanical load with adrive output signal based on a reference signal, the drive output signalinducing a first electrical quantity at the electromechanical load, thedriver circuitry comprising: a function block configured, based on saidfirst electrical quantity, to digitally determine an adjustment signalindicative of a second electrical quantity which would be induced at atarget output impedance of the driver circuitry due to said firstelectrical quantity; and a driver configured to generate the driveoutput signal based on the reference signal and the adjustment signal tocause the drive output signal to behave as if an output impedance of thedriver circuitry has been adjusted to comprise the target outputimpedance.

According to a thirteenth aspect of the present disclosure, there isprovided driver circuitry for driving an electromechanical load with adrive output signal based on a reference signal, the driver circuitryconfigured to digitally control the drive output signal based on thereference signal to cause the drive output signal to behave as if anoutput impedance of the driver circuitry has been adjusted to comprise adefined or predetermined target output impedance.

According to a fourteenth aspect of the present disclosure, there isprovided driver circuitry for driving an electromechanical load with adrive output signal based on a reference signal, the driver circuitryconfigured to digitally control the drive output signal based on thereference signal and (a feedback signal indicative of) an electricalquantity at the electromechanical load (responsive to the drive outputsignal) to cause the drive output signal to behave as if an outputimpedance of the driver circuitry has been adjusted to comprise adefined or predetermined target output impedance.

Method and computer program aspects are envisaged corresponding to thecircuitry aspects. IC chip, control system, haptic system and hostdevice system aspects are envisaged for each of the driver circuitryaspects, analogous to those specified above in relation to the firstaspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example only, to the accompanyingdrawings, of which:

FIG. 1 is a schematic diagram of an equivalent circuit representingdriver circuitry driving an LRA under open loop control;

FIG. 2 is a schematic diagram of an equivalent circuit corresponding tothat of FIG. 1 , but including a target output impedance;

FIGS. 3A to 3E are schematic diagrams of equivalent circuitsrepresenting particular configurations of the target output impedance ofFIG. 2 ;

FIG. 4 is a schematic diagram of driver circuitry according to anembodiment;

FIG. 5 is a schematic diagram of driver circuitry according to anembodiment;

FIG. 6 is a schematic diagram of part of the driver circuitry of FIG. 5, according to a detailed implementation;

FIG. 7 is a schematic diagram of an example implementation of part ofthe driver circuitry of FIG. 4 for use in a current-drive mode ofoperation;

FIG. 8 presents a series of graphs useful for understanding benefits andcapabilities of the driver circuitry disclosed herein;

FIG. 9A is a schematic diagram of the modified driver circuitry of FIG.2 ;

FIG. 9B is a schematic diagram corresponding to the modified drivercircuitry of FIG. 2 but using current source control rather than voltagesource control; and

FIG. 10 is a schematic diagram of a host device according to anembodiment.

DETAILED DESCRIPTION

Before introducing embodiments, the operation of an LRA will beconsidered in more detail. As above, an LRA is merely one convenienttype of electromechanical load or electromechanical device, particularlyof interest when haptic systems are considered. It will be understoodthat the teachings herein apply to driving electromechanical loads ingeneral, for example other types of actuator useful in haptic systems.

When an LRA is driven with a voltage across its two electrical terminalsa current flows through or is drawn by the voice coil (an inductor)producing an electromotive force (EMF) on the moveable magnetic mass andthus controlling its motion. The moveable magnetic mass is connected toa spring which thus also affects its motion. The moving magnetic mass inturn produces a back EMF (bemf) voltage proportional to its velocity,which is reflected at the electrical terminals. The setup is akin to adriven (damped) harmonic oscillator.

With this in mind it is helpful to consider the driving of an LRA inelectrical terms. FIG. 1 is a schematic diagram of an equivalent circuit1 of driver circuitry driving an LRA under open loop control, along witha graph and equations useful for understanding its operation.

The equivalent circuit 1 of FIG. 1 comprises an AC voltage source(voltage amplifier) 10, modelling the driver circuitry, connected to anLRA (electromechanical) load 20, modelling the LRA. The LRA load 20 willbe referred to merely as LRA 20 herein for simplicity. The AC voltagesource 10 produces a reference voltage ref (driving signal) whichappears across the LRA 20 and induces a load current iload to be drawnby the LRA 20.

The LRA 20 comprises a coil impedance zcoil, which models the voicecoil, and a mechanical impedance zmech, which models the moveable massand spring arrangement. The coil impedance zcoil is modelled as aninductance le in series with a resistance re. The mechanical impedancezmech appears in series with the coil impedance zcoil and is modelled asa parallel network of a capacitance cmes, an inductance Ices and aresistance res. The capacitance cmes models the magnetic mass, theinductance Ices models the spring and the resistance res models themechanical damping. The bemf voltage appears across the mechanicalimpedance zmech as indicated (recall it is induced by the movingmagnetic mass).

The user haptic experience is defined by sensing the motion of themoving mass, and in particular the force generated by its acceleration(recall Newton's second law, F=ma). It is desirable therefore to controlthe acceleration of the moving mass, or a proxy such as the position orvelocity of the moving mass (based on which the acceleration can becontrolled). It is desirable in particular to control the accelerationto produce one or more of: a) crisp haptic effects (e.g. fastacceleration and braking of the mass, e.g. to simulate clicks or buttonpresses); b) wide bandwidth effects (e.g. for audio-to-haptics or toreplicate textures); and c) consistent effects (e.g. from LRA to LRA, orover changing environmental conditions). It is desirable to sharpen theonset of the LRA's response to haptic input pulses and to reduce ringingof the LRA (and e.g. smartphone screen in the context of surfaceaudio/haptic applications) after the haptic input pulse has stopped.

The bemf voltage is proportional to the velocity of the moving mass asmentioned above, however the control by the driver circuitry as in FIG.1 controls the reference voltage ref rather than the bemf voltageitself. Such open loop voltage drive produces the highly resonantperformance indicated by the graph in FIG. 1 .

In particular, the ref-to-bemf transfer function (bemfTF) of the drivenLRA load is set by the voltage divider defined by zcoil and zmech asapparent from the equivalent circuit of FIG. 1 . Similarly, the loadcurrent transfer function (iloadTF) is set by the series connection ofzcoil and zmech. These relationships are expressed in the equations inFIG. 1 , where zmech is represented as zBemf and zcoil is represented aszCoil.

Because the mechanical system is resonant with high Q (quality factor),and the mechanical impedance is a lot smaller than the coil impedance(i.e. zmech>>zcoil), the ref-to-bemf transfer function bemfTF has a verynarrow bandwidth. The velocity effectively follows zmech (expressed aszBemf) away from resonance (where zbemf<<zcoil) and such driving is inpractice only useful for simple vibration effects. To produce muchacceleration the vibration frequency needs to be close to the resonantfrequency.

The present inventors have considered modifying the AC voltage source 10by adjusting its output impedance to affect control of the LRA 20 and inparticular the bemf voltage.

FIG. 2 is a schematic diagram of an equivalent circuit 2 correspondingto equivalent circuit 1, except that a target output impedance 30 hasbeen interposed between the AC voltage source (voltage amplifier) 10 andthe LRA 20. The combination of the AC voltage source 10 and the targetoutput impedance 30 is then referred to as modified driver circuitry 40,whose drive output signal, drive voltage dry (driving signal), isprovided at an output node 42 (between node 42 and ground) locatedbetween the modified driver circuitry 40 and the LRA load 20 to drivethe LRA load 20 as indicated, based on the reference signal ref. Notethat the target output impedance 30 is referred to in FIG. 2 as a“virtual” impedance (as discussed in more detail later), and is providedalong a current path which carries the load current iload which flowsthrough or is drawn by the LRA 20 based on the drive voltage dry (driveoutput signal).

It is assumed in FIG. 2 that the AC voltage source 10 itself is ideal(i.e. with zero output impedance). Thus, the target output impedance 30may be considered the output impedance of the modified driver circuitry40. Of course, in a practical implementation the AC voltage source 10may be non-ideal (i.e. with some, albeit small, output impedance of itsown). In that case, the target output impedance 30 may be consideredpart of (e.g. a substantial or dominant part of) the output impedance ofthe modified driver circuitry 40. This may be represented in FIG. 2 bysome additional impedance (not shown) in series between the AC voltagesource 10 and the target output impedance 30 which may be taken accountof when determining a desired target output impedance 30.

The target output impedance 30 is presented in equivalent-circuit formin the equivalent circuit 2 as comprising a series resistance ser_r, aseries capacitance ser_c, a series inductance ser_l and a parallelnetwork of impedances, connected together in series. The parallelnetwork of impedances comprises a parallel resistance par_r, a parallelcapacitance par_c and a parallel inductance par_l, connected together inparallel.

The target output impedance 30 is presented in FIG. 2 as comprising allof these impedances as one example of how complex the target outputimpedance may be. However, the inventors have considered variants inwhich some of these impedances are effectively or actually not present(e.g. are shorted or removed) to define a less complex target outputimpedance 30.

A number of such variants are presented in FIGS. 3A to 3E as examples.Each of the variants may be considered to be a particular configurationof the target output impedance 30 of FIG. 2 .

FIG. 3A is a schematic diagram of an equivalent circuit of a targetoutput impedance 30A, being a variant of the target output impedance 30which comprises only the series resistance ser_r. If the target outputimpedance 30 in FIG. 2 were replaced with (or configured to form) thetarget output impedance 30A, and the series resistance ser_r were giventhe value −re (a negative resistance), it can be seen from FIG. 2 thatthe series resistance ser_r would then ‘cancel out’ the seriesresistance re of the coil impedance zcoil (as if neither resistance werepresent). The implementation of such a negative resistance will beexplained later.

At low frequencies (e.g. <1 kHz) the inductance le of the coil impedancezcoil may be negligible and be assumed to be shorted. In that case, asapparent from FIG. 2 , the reference voltage ref would appear across themechanical impedance zmech such that the bemf voltage follows thereference voltage ref. This enables control of the bemf voltage itselfwith the reference voltage ref, and thus of the velocity of the magneticmass of the LRA (and hence its acceleration and the force or hapticeffect generated by that acceleration). For example, the referencevoltage ref may take the form of a haptic signal with the modifieddriver circuitry 40 having the target output impedance 30A enablingwider bandwidth control of the velocity (or position or acceleration) ofthe LRA mass to create interesting haptic effects.

Incidentally, it is noted in FIG. 3A that it may be desirable to givethe series resistance ser_r a value which is approximately (e.g. up to5% or around 1% off) −re. This arises from an example where, in order toachieve a damping factor zeta_lra of the LRA equal to 1 (i.e. for theLRA to be critically damped), it was determined that ser_r should be setto have a value which is approximately 99% of −re. The skilled personwill appreciate that in a given application a value of ser_r whichachieves critical damping could be found. Where the value −re is usedlater herein, it will be understood that this value could be adjusted toachieve critical damping in some arrangements. Critical damping isdesirable to sharpen the onset of haptic pulses in the LRA (driven byway of the reference signal ref) and to reduce ringing of the LRA afterthe haptic pulse in the reference signal ref has stopped.

FIG. 3B is a schematic diagram of an equivalent circuit of a targetoutput impedance 30B, being a variant of the target output impedance 30which comprises only the series resistance ser_r and the seriesinductance ser_l. If the target output impedance 30 in FIG. 2 werereplaced with (or configured to form) the target output impedance 30B,and the series resistance ser_r and series inductance ser_l givenrespective values −re and −le (negative resistance and inductance), itcan be seen from FIG. 2 that the target output impedance 30B would then‘cancel out’ the coil impedance zcoil, even where the inductance lecannot be ignored at low frequencies. As apparent from FIG. 2 , thereference voltage ref would again appear across the mechanical impedancezmech with the bemf voltage following the reference voltage ref (butover a larger bandwidth than with the target output impedance 30A).

FIG. 3C is a schematic diagram of an equivalent circuit of a targetoutput impedance 30C, being a variant of the target output impedance 30in which the series capacitance ser_c has been omitted. In this case, itcan be appreciated that the types of impedance and their interconnectionin the target output impedance 30C somewhat “mirror” those of the LRA20.

If the target output impedance 30 in FIG. 2 were replaced with (orconfigured to form) the target output impedance 30C, with the seriesresistance ser_r and the series inductance ser_l given respective values−re and −le (negative resistance and inductance), again those componentswould cancel out the coil impedance zcoil as for the target outputimpedance 30B. The parallel RLC section of the target output impedance30C, i.e. the parallel resistance par_r, parallel capacitance par_c andparallel inductance par_l, may then be used to cause the mechanicalimpedance zmech to appear differently in electrical terms to the ACvoltage source 10, i.e. to effectively synthesise a desired LRA load.

FIG. 3D is a schematic diagram of an equivalent circuit of a targetoutput impedance 30D, being a variant of the target output impedance 30which comprises only the series resistance ser_r and the seriescapacitance ser_c. If the target output impedance 30 in FIG. 2 werereplaced with (or configured to form) the target output impedance 30D,and the series resistance ser_r were given the value −re (a negativeresistance) as before, again the series resistance ser_r would ‘cancelout’ the series resistance re of the coil impedance zcoil (as if neitherresistance were present). The series capacitance ser_c and theinductance le of the coil impedance zcoil then effectively form an LCtank in series with the mechanical impedance zmech, which can increasedamping to bring the magnetic mass of the LRA to a stop faster.

FIG. 3E is a schematic diagram of an equivalent circuit of a targetoutput impedance 30E being a variant of the target output impedance 30in which it comprises only the series resistance ser_r as in FIG. 3A butwhere the series resistance ser_r is given a value which is much bigger(e.g. >10 times bigger) than the resistance res of the mechanicalimpedance zmech (e.g. ser_r>>res and even ser_r>>re). In thisarrangement, the position of the magnetic mass of the LRA isproportional to the reference voltage ref at frequencies below resonanceand its acceleration is proportional to the reference voltage ref atfrequencies above resonance.

Against this backdrop, FIG. 4 is a schematic diagram of driver circuitry40A for driving the LRA 20, according to an embodiment. It will becomeapparent that the driver circuitry 40A implements a number of controlloops.

The driver circuitry 40A comprises a function block 50, a driver 60 anda controller 70. The controller 70 is optional—it may for example beprovided separately from the driver circuitry 40A (the function block 50and driver 60) in some applications. The combination of the functionblock 50, driver 60 and controller 70 corresponds to the modifiedcircuitry and thus outputs its drive output signal to the LRA 20 at anoutput node 42 for consistency with FIG. 2 . The driver circuitry 40A isshown connected at the output node 42 to drive the LRA 20 forconvenience, but it will be understood that the driver circuitry 40Aneed not actually comprise the LRA 20 (the LRA 20 may be providedseparately for connection to the driver circuitry 40A).

In general, for convenience, digital signals will be denoted in thefollowing using block capitals (e.g. MON) and analogue signals will bedenoted in lower case (e.g. mon).

The function block 50 is configured to generate a (digital) controlsignal CS as a function of a (digital) reference signal RS and a(digital) monitor signal MON (which—although not shown—may be generatedfrom a corresponding analogue monitor signal mon). The reference signalRS is generated by the controller 70 and is intended for controlling theLRA 20. For example, the reference signal RS may exhibit haptic pulsesto be used to control the LRA 20. The reference signal RS may beindicative of (e.g. proportional to, directly proportional to, or have apredefined, defined, or linear relationship with) an intended mechanicalperformance of the LRA 20 (electromechanical load). In this sense, thecontroller 70 and the reference signal RS may be compared with thedriver circuitry and the reference voltage ref, respectively.

The driver 60 is configured to convert the control signal CS into an(analogue) drive output signal dos (a voltage signal) which is outputvia the output node 42 to drive the LRA 20. The LRA 20 draws the loadcurrent iload due to the drive output signal dos. The load current iloadis thus the current (flowing) through the LRA 20. One or more of thereference signal RS (including any signal based on which the referencesignal RS is generated), control signal CS and drive output signal dosmay be referred to as an actuating signal. It is recalled that the LRA20 is an example of an electromechanical load or electromechanicaldevice. The driver 60 may comprise a digital-to-analogue converter (notshown) to convert the digital control signal CS into an analogue controlsignal cs and an amplifier (also not shown) to amplify the analoguecontrol signal cs to generate the analogue drive output signal dos.

The monitor signal MON may comprise a current monitor signal IMON whichis indicative of (e.g. proportional to, directly proportional to, or hasa predefined, defined or linear relationship with) the load currentiload flowing through or drawn by the LRA 20. The monitor signal MON may(additionally or alternatively) comprise a voltage monitor signal VMONwhich is indicative of (e.g. proportional to, directly proportional to,or has a predefined, defined or linear relationship with) a voltageinduced across the LRA 20 due to the current flowing through the LRA 20(effectively, the drive output signal dos where this is applied simplyacross the LRA 20). As such, the driver circuitry 40A may comprisemonitoring circuitry 80 to monitor the current flowing through (andoptionally also the voltage across) the LRA 20 and generate the monitorsignal MON (or its analogue equivalent mon).

It is emphasised that the monitoring circuitry 80 need not be part of(e.g. housed within) the LRA 20 and indeed may be considered separatefrom the LRA 20 so that the LRA may be provided without needing anysensing technology (i.e. it may be a “sensorless” LRA). This will becomemore apparent in connection with FIG. 5 described below.

As indicated schematically in respect of the controller 70, thecontroller 70 is configured to generate the reference signal RS based ona drive input signal DIS. The drive input signal DIS may be generatedwithin the controller 70 or received from a separate system orcontroller (e.g. from an applications processor). The drive input signalDIS may be generated within the controller 70 based on one or morereceived signals, e.g. from a separate system or controller (e.g. froman applications processor).

The controller 70 may be configured to receive the monitor signal MON orpart thereof, and to control one or more of its signals based on themonitor signal MON or part thereof. For example, the controller 70 maybe configured to receive the current monitor signal IMON and/or thevoltage monitor signal VMON and to control one or more of the signalswhich it generates based on the current monitor signal IMON and/or thevoltage monitor signal VMON.

The controller 70 may be configured to generate the reference signal RSbased on the current monitor signal IMON and/or the voltage monitorsignal VMON. The current monitor signal IMON and/or the voltage monitorsignal VMON may for example be indicative of the performance of the LRA20, such as its mechanical performance. The current monitor signal IMONand the voltage monitor signal VMON may be used together to assess e.g.the onset of the LRA 20 response to haptic input pulses (expressed bythe drive input signal DIS and/or the reference signal RS) or the degreeof ringing of the LRA after the haptic input pulse has stopped. Thecurrent monitor signal IMON and the voltage monitor signal VMON may beused together to assess e.g. the resonant frequency f0, quality factorQ, impedance and/or operational state (including failure states) of theLRA 20. The current monitor signal IMON and the voltage monitor signalVMON may for example be indicative of the effectiveness of the current(present or existing) configuration of the target output impedance 30,and indicate how that configuration should be varied to meet aperformance target. The implementation of the target output impedance 30in the driver circuitry 40A is described in more detail below.

In this way, a first control loop may be formed in which the driveoutput signal dos is controlled based on a drive input signal DIS. Insuch a control loop it can be understood that the monitor signal MON(the current monitor signal IMON and/or the voltage monitor signal VMON)serves as a feedback signal for feedback control (by the controller 70)of the reference signal RS and thus also of the control signal CS andthe drive output signal dos. This feedback control may be used to keepthe performance of the LRA 20 (as indicated by the current monitorsignal IMON and/or the voltage monitor signal VMON, e.g. its mechanicalperformance) within performance limits.

The first control loop may also incorporate feedforward control (by thecontroller 70) of the reference signal RS and thus also of the controlsignal CS and the drive output signal dos. There are a multitude ofpossibilities, for example using high-pass filtering to removelower-frequency components which may lead to low-efficiency driving ofthe LRA 20, or low-pass filtering to handle erroneous sounds that can beemitted by some real life mechanical integrations, or anaudio-to-haptics analyser which converts audio content to hapticcontent. These are of course simply examples.

A second control loop may also be considered to be present, in which thedrive output signal dos is controlled based on the monitor signal MON(the current monitor signal IMON and/or the voltage monitor signalVMON). In such a control loop it can be understood that the monitorsignal MON (in particular, the current monitor signal IMON) serves as afeedback signal for feedback control (by the function block 50) of thecontrol signal CS and thus also of the drive output signal dos. Thecontrol by the function block 50 will be described in more detail below.

A third control loop may also be considered to be present, in which thefunctionality of the function block 50 (described below) is controlledbased on the monitor signal MON (the current monitor signal IMON and/orthe voltage monitor signal VMON). In such a control loop it can beunderstood that the monitor signal MON (in particular, the currentmonitor signal IMON and the voltage monitor signal VMON) serves as afeedback signal for feedback control (by the controller 70) of aconfiguration signal CONFIG which is supplied to the function block 50to define or affect or control its operation. As above, the control bythe function block 50 will be described in more detail below.

A fourth control loop may also be considered to be present, in which thefunction of the driver 60 is controlled based on the monitor signal MON(the current monitor signal IMON and/or the voltage monitor signalVMON). In such a control loop it can be understood that the monitorsignal MON (in particular, the voltage monitor signal VMON) serves as afeedback signal for feedback control (by the driver 60) of the driveoutput signal dos e.g. so that the drive output signal dos has a definedor predefined (e.g. linear, proportional or directly proportional)relationship with the control signal CS. This control may for exampleact to achieve linear operation of the driver 60 (which may beconsidered an amplifier).

It will become apparent that it may be desirable to operate the variouscontrol loops with different (relative) latencies. More particularly,for one or more of the control loops it may be desirable to have lowlatencies, e.g. so that analogue operation is closely simulated (acrossa bandwidth of interest—which may be e.g. a haptic or audio bandwidth asmentioned later), whereas for one or more others of the control loops itmay be acceptable (or desirable, with power consumption and complexityin mind) to operate with higher latencies.

For example, the second control loop may have a lower latency than thefirst and/or third control loop. The fourth control loop may have alower latency than the first and/or third control loop. The secondcontrol loop may have the same or substantially the same latency as thefourth control loop. The latencies of the control loops may be definedby sample rates (update rates, response rates) of respective digitalsignals of the control loops, as will become more apparent in connectionwith FIG. 5 (described below). The term latency here may thus describehow quickly (e.g. at what rates, speeds, or frequencies) a particularcontrol loop responds to disturbances or control inputs.

The above control loops may be referred to as (or be considered toencompass) control paths or control systems or control networks. Each ofthe control loops may incorporate one or more of feedback control,feedforward control and open loop control.

In overview, the function block 50 controls a difference between thecontrol signal CS and the reference signal RS so as to simulate thepresence of an (analogue) target output impedance of the drivercircuitry 40A corresponding to the target output impedance 30 of FIG. 2(whose impedance value may be set to configure the target outputimpedance as e.g. any of the target output impedances 30A to 30E). Forconvenience, the simulated target output impedance will be referred tosimply as the target output impedance 30.

Because the function block 50 controls the difference between thecontrol signal CS and the reference signal RS in the digital domain(i.e. digitally, using digital signals and digitaloperations/calculations), the relationship between the control signal CSand the reference signal RS can be configured (e.g. over time) to defineand/or adjust the configuration of the target output impedance 30 (e.g.which of the target output impedances 30A to 30E is being used). In thisway, the response of the LRA 20 to the reference signal RS can becontrolled enabling the haptic effect (expressed by the reference signalRS) to be controlled in the case of a haptic system.

The function block 50 is configured, based on the monitor signal MON (inparticular, the current monitor signal IMON), to digitally determine anadjustment signal AS indicative of a voltage signal which would beinduced across the target output impedance 30 of the driver circuitry40A by the load current iload (i.e. if the load current were to flowthrough the target output impedance 30). In effect, the function block50 is configured to digitally determine (e.g. calculate) the adjustmentsignal AS based on the load current iload. The driver 60 is thenconfigured to generate the drive output signal dos based on thereference signal RS and the adjustment signal AS (or based on thecontrol signal CS, which itself is generated based on the referencesignal RS and the adjustment signal AS) to cause the drive output signaldos to behave as if an output impedance of the driver circuitry has beenadjusted to comprise the target output impedance 30.

In more detail, the function block 50 is configured, based on themonitor signal MON (in particular, the current monitor signal IMON), tocontrol a relationship or difference between the control signal CS andthe reference signal RS. In particular, the function block 50 controlsthe relationship so that the drive output signal dos (when driving theLRA 20) has a target behaviour in which the drive output signal dosbehaves, relative to an expected analogue drive output signal expectedto be generated with the control signal CS being the reference signal RS(i.e. when CS=RS), as if the output impedance of the driver circuitryhas been configured or adapted or adjusted to comprise (or simplycomprises) a target output impedance such as the target output impedance30 (e.g. configured to form any of the variants 30A to 30E). Theexpected analogue drive output signal is expected to be generated if thedriver 60 generates the drive output signal dos based on the referencesignal RS without the adjustment signal AS (effectively without thecontrol of the second control loop, which may be taken to comprise theadjustment signal AS).

Thus, the function block 50 adjusts the control signal CS relative tothe reference signal RS so that driver circuitry 40A behaves as if itsoutput impedance (measured at node 42) includes the target outputimpedance 30 (whereas it otherwise would not, i.e. when CS=RS). Forexample, where CS=RS the driver circuitry 40A may be able to operatewith zero output impedance (measured at node 42) due to the operation ofthe driver 60, in which case the function block 50 adjusts CS relativeto RS based on IMON so that the output impedance of the driver circuitry40A is substantially equal to the target output impedance.

In this sense, the function block 50 simulates or emulates the presenceof the target output impedance 30 by making an adjustment (by way ofadjustment signal AS) in the signal path between the controller 70 andthe driver 60 so that the output impedance of the driver circuitry 40Aappears to (and indeed in effect does) comprise the target outputimpedance 30. The target output impedance 30 in these terms may beconsidered a “virtual” impedance as mentioned earlier in that it is notimplemented by providing analogue discrete passive impedance components,but by virtue of signal adjustments determined in the digital domain(i.e. made or determined digitally).

For example, the function block 50 may be configured to receive and/orstore one or more configuration values which define the target outputimpedance, and thus govern how the control signal CS is generated as afunction of the reference signal RS and the current monitor signal IMON.The configuration values may be set based on the (digital) configurationsignal CONFIG received from the controller 70, as indicated in FIG. 4 ,by virtue of the third control loop.

Incidentally, the (digital) configuration signal CONFIG may becontrolled by a separate impedance control signal (not shown), e.g.received from a separate system, either instead of or in addition tocontrol by virtue of the third control loop. The configuration of thetarget output impedance may thus vary with (or be controlled by or beset by) the separate impedance control signal in this way.

Returning to FIG. 4 , a useful example to appreciate the “virtual”aspect of the target output impedance 30 is where the target outputimpedance 30 is configured to form the target output impedance 30A withits resistance ser_r having the value −re (i.e. a negative resistance)as mentioned earlier. Looking at FIG. 2 , such a target output impedance30A (negative resistance) could be expected to have a voltage rise(rather than drop) across it in the direction from the driver circuitry40 to the LRA load 20 which is defined by the product of the resistancevalue re and the load current iload flowing through the LRA load 20 inthat direction (recall Ohm's law, V=IR).

Thus, in this example the function block 50 simulates the presence ofthe target output impedance 30A between the controller 70 and the driver60 by adding an amount represented by the adjustment signal AS (based onthe product of the resistance value re and the load current iloadflowing through or drawn by the LRA load 20, as indicated by the currentmonitor signal IMON) to the reference signal RS to form the controlsignal CS (i.e. CS=RS+AS), so that the output impedance of the drivercircuitry 40A appears to (and in effect does) comprise the target outputimpedance 30A.

In this case, the adjustment signal AS could be considered to be afunction of iload*re or IMON*re. In this way, the function block 50enables a negative resistance to be implemented digitally. The CONFIGsignal may for example simply provide the function block 50 with thevalue −re (or re) to define the series resistance ser_r, possibly alongwith other configuration values which define the target output impedance30 as being of the form of (or configured as) the target outputimpedance 30A (rather than e.g. the target output impedance 30C).

The drive output signal dos is a voltage signal which appears across theLRA 20. The target behaviour can then be defined by how a voltage levelof the drive output signal dos varies when driving the LRA 20 (or avoltage across the LRA 20) with the current flowing through the LRA 20(i.e. the load current iload).

Incidentally, in the FIG. 4 embodiment the control signal CS, referencesignal RS, adjustment signal AS and monitor signal MON are presented asbeing digital signals as a convenient implementation which enables thefunction block 50 to be considered a (fully) digital block. However, thecontrol signal CS, adjustment signal AS and reference signal RS couldfor example be replaced with analogue equivalent signals cs, as and rs,respectively. In this case, the function block 50 may digitally (e.g. bycalculation or using a look-up table) work out how to adjust theanalogue control signal cs relative to the analogue reference signal rs(i.e. what the adjustment signal as should be) to simulate or emulatethe presence of the target output impedance 30. For example, thefunction block 50 may digitally generate (e.g. by calculation or using alook-up table, followed by digital-to-analogue conversion) a suitableanalogue adjustment signal as to be added in the analogue domain to theanalogue reference signal rs to generate the analogue control signal cs.It will be appreciated that by digitally working out how to adjust theanalogue control signal cs relative to the analogue reference signal rsit is possible to implement the target output impedance 30 (including inthe form of the target output impedance 30A with a negative resistance)in an efficient and highly adaptable manner.

For convenience, the example using the digital control signal CS,digital adjustment signal AS, digital reference signal RS and digitalmonitor signal MON will be carried forwards.

It was mentioned earlier that there may be one or more analogue outputimpedances (e.g. discrete or parasitic components) present in the drivercircuitry 40A. The target output impedance 30 may be configured to takethis into account. For example, if there is some analogue (positive)resistance of magnitude R1 (not shown) in the output impedance of thedriver circuitry 40A, and it is desired that the output impedance of thedriver circuitry 40A have an overall resistance of magnitude −R2 (anegative resistance), then the target output impedance 30 (assuming theFIG. 3A configuration) may be configured to take this into account bysetting ser_r=−(R1+R2), i.e. configured to compensate for (or allow for)the analogue impedance R1.

As another example, the function block 50 may be configured to controlan analogue variable impedance (a discrete component—not shown) in thecurrent path of the load current iload, such as between the output node42 and the driver 60, so that the output impedance of the drivercircuitry 40A is controlled or adjusted in part with the variableimpedance (an actual discrete impedance component). Again, the targetoutput impedance 30 may be configured to take this into account, i.e.adjusted or configured to compensate for (or allow for) the variableimpedance. For example, an impedance formed by (equivalent to) acombination of the variable impedance and the target output impedance 30may be controlled.

FIG. 5 is a schematic diagram of driver circuitry 40B for driving theLRA 20, as a detailed example implementation of the driver circuitry40A. Like elements and signals are denoted with like reference signs andduplicate description is omitted. The function block 50 is referred toas function block 50A in the FIG. 5 implementation.

It will become apparent that in the present detailed implementation somedigital signals have a relatively high sample (update) rate and otherdigital signals have a relatively low sample (update) rate, and this isindicated with the suffixes “(H)” and “(L)”, respectively. In this way,some signals and the corresponding control loops may be considered“fast” (or high bandwidth, or low latency) and some signals and thecorresponding control loops may be considered “slow” (or low bandwidth,or high latency), as mentioned earlier. The signals with the low samplerate could be considered to have the same sample rate as one another,and the signals with the high sample rate could similarly be consideredto have the same sample rate as one another, but this is not essential.Also, the various sample rates could be varied depending on theapplication and e.g. over time.

The function block 50A of the driver circuitry 40B comprises acurrent-monitoring ADC 510, a voltage-monitoring ADC 520, acurrent-monitoring decimator 530, a voltage-monitoring decimator 540, anadjustment signal (AS) determiner 550, an adder 560 and a clipper 570.The function block 50A is a digital block (except for analogue front-endportions of the ADCs 510 and 520), and may be implemented using“hardwired” circuitry, logic gates and/or a processor executing acomputer program. For example, in some arrangements the function block50A may be implemented as part of the controller 70, which may be aprocessor or microprocessor such as a digital signal processor (DSP). Assuch, the division of the function block 50A into interconnectedcomponent parts in FIG. 5 may be considered schematic and useful forunderstanding its function.

In some arrangements the controller 70 may be considered part of thedriver circuitry 40B, e.g. provided as part of the same integratedcircuitry as other elements of the driver circuitry 40B. In otherarrangements, the controller 70 may be considered separate from thedriver circuitry 40B, e.g. provided as separate integrated circuitryfrom integrated circuitry comprising other elements of the drivercircuitry 40B.

It is assumed here that the monitoring circuitry 80 is configured tomonitor the current flowing through the LRA 20 and output an analoguecurrent-monitoring signal imon, and also to monitor the voltage acrossthe LRA 20 and output an analogue voltage-monitoring signal vmon. It isalso emphasised that the monitoring circuitry 80 may be separate fromthe LRA 20, with the LRA 20 in this case shown as being connected acrossterminals 82 and 84 of the monitoring circuitry 80 (which terminals 82and 84 may be considered terminals of the driver circuitry 40B). Thus,the driver circuitry 40B including the monitoring circuitry 80 (butexcluding the LRA 20) could be implemented as integrated circuitry, forexample on an IC chip, with the terminals 82 and 84 being (external)terminals of the integrated circuitry.

As an example, the monitoring circuitry 80 may comprise a resistor (notshown) connected in series with the LRA 20 such as between nodes 42 and82 (whose known resistance is taken into account when assessing theresistance re of the coil impedance zcoil), with a voltage across thatresistor being proportional to the load current iload flowing throughthe LRA 20 and thus forming the current-monitoring signal imon. Thevoltage-monitoring signal vmon may be formed by a load voltage vloadtaken across the LRA 20, e.g. across terminals 82 and 84. Of course,there are other ways to obtain the signals vmon and imon in respect ofthe LRA 20.

The current-monitoring ADC 510 is connected to receive the analoguecurrent-monitoring signal imon and output a corresponding digitalcurrent-monitoring signal IMON (H), i.e. having a high sample rate. Thecurrent-monitoring decimator 530 is connected to receive thecurrent-monitoring signal IMON (H) and to output a corresponding digitalcurrent-monitoring signal IMON (L), i.e. having a low sample rate. Thevoltage-monitoring ADC 520 is connected to receive the analoguevoltage-monitoring signal vmon and output a corresponding digitalvoltage-monitoring signal VMON (H), i.e. having a high sample rate. Thevoltage-monitoring decimator 540 is connected to receive thevoltage-monitoring signal VMON (H) and to output a corresponding digitalvoltage-monitoring signal VMON (L), i.e. having a low sample rate. Adecimator in this sense acts to reduce the sample rate between its inputand output signals, e.g. by outputting one input sample per severalinput samples or averaging successive groups of samples.

The controller 70 is connected to receive the signals IMON (L) and VMON(L), the AS determiner 550 is connected to receive the signal IMON (H),and the driver 60 is connected to receive one or both of the signalsvmon and VMON (H). It is assumed that the controller comprises aninterpolator 710 which converts a digital reference signal RS (L), i.e.at a low sample rate, into a corresponding digital reference signal RS(H) at a high sample rate. An interpolator in this sense acts toincrease the sample rate between its input and output signals,generating new samples by interpolation/estimation.

In overview, the function block 50A of the driver circuitry 40B isconfigured to determine, based on the monitor signal MON and one or moreconfiguration values which define the target output impedance 30, theadjustment signal AS (H) to be applied to the reference signal RS (H) toform the control signal CS (H) and cause the drive output signal doswhen driving the LRA 20 to have the target behaviour, and to generatethe control signal CS (H) by applying the adjustment signal AS (H) tothe reference signal RS (H). Note this forms part of the second controlloop (which is fast, with low latency, using high sample (update) ratedigital signals) so that the drive output signal dos behaves (over awide bandwidth) as if the target output impedance 30 had beenimplemented in analogue form.

In detail, the AS determiner 550 is configured to generate theadjustment signal AS (H) based on the signal IMON (H) in a form to beadded to the reference signal RS (H) to form the control signal CS (H).The adder 560 is configured to generate the control signal CS (H) byadding adjustment signal AS (H) to the reference signal RS (H). Thecontrol signal CS (H) is thus generated to have a defined or predefined(e.g. substantially linear, proportional or directly proportional)relationship with a summation of the adjustment signal AS (H) and thereference signal RS (H).

Effectively, the AS determiner 550 determines (e.g. by calculation orusing a look-up table) a voltage which would be induced across thetarget output impedance 30 if the current flowing through the LRA 20were to flow through the target output impedance 30, and generates theadjustment signal AS (H) to express this voltage so that adding theadjustment signal AS (H) to the reference signal RS (H) produces thecontrol signal CS (H). Thus, the adjustment signal AS (H) may beconsidered as indicative of (e.g. proportional to, directly proportionalto, or having a predefined, defined or linear relationship with) theload voltage vload which would be induced across the target outputimpedance 30 if the load current iload flowing through the LRA 20 wereto flow through the target output impedance 30. In this way, the controlsignal CS (H) and thus the drive output signal dos will respond to theload current iload as if the output impedance of the driver circuitry40B has been configured to comprise the target output impedance 30.

The clipper 570 serves to clip (i.e. keep within limits) the values ofthe control signal CS (H), e.g. so that its values are within a linearoperation range of the driver 60 (e.g. of a DAC and/or analogueamplifier of the driver 60). The driver 60 is configured to control thedrive output signal dos so that its voltage level has a defined orpredefined (e.g. substantially linear, proportional or directlyproportional) relationship with the control signal CS (H), by virtue ofone or both of the signals VMON (H) and vmon as indicated, as part ofthe fourth control loop (which is fast similarly to the second controlloop). The clipper 570 is optional in some arrangements.

Note that the controller 70 is connected to receive the monitor signalMON in the form of the digital current-monitoring signal IMON (L) andthe voltage-monitoring signal VMON (L), both with the low sample rate.As such, the first control loop generates the reference signal RS (L)based on the drive input signal DIS (L) as indicated, acting as arelatively slow control loop. The interpolator 710 converts thereference signal RS (L) into the corresponding reference signal RS (H),for use in the second and fourth control loops which are fast controlloops as already mentioned. Further, the third control loop generatesthe configuration signal CONFIG (L), i.e. with the low sample rate, foruse by the function block 50A (in particular, the AS determiner 550) todefine the target output impedance 30, i.e. by way of one or moreconfiguration values, acting as a relatively slow control loop.

The first and third control loops may for example only need to respondto relatively slow (low frequency) disturbances, e.g. changes intemperature of the LRA 20. On the other hand, the second and fourthcontrol loops may need very low latencies to simulate or emulateanalogue performance (over a given bandwidth).

Looking further at the first and third control loops, it will beappreciated that the controller 70 has access to the monitor signal MONin the form of the digital current-monitoring signal IMON (L) and thevoltage-monitoring signal VMON (L), as mentioned earlier. Based on thesesignals, the controller 70 may in some arrangements be configured todetermine or estimate partly or fully the configuration of the LRA 20,e.g. to determine impedance values (or estimates thereof) of some or allof (see FIG. 2 ) the values re, le, cmes, Ices and res. The analysis ofthe digital current-monitoring signal IMON (L) and thevoltage-monitoring signal VMON (L) may also enable a determination orestimation of the resonant frequency f0 or quality factor Q of the LRA20.

This information can be used to define or update the configuration ofthe target output impedance 30 via the CONFIG (L) signal and/or tocontrol parameters of the reference signal RS (L). An example may bedetermining or estimating the value of the coil resistance re (see FIG.2 ) so as to set or update (e.g. improve) the value used for the seriesresistance ser_r of the target output impedance 30 (e.g. in the case ofthe FIG. 3A configuration). Another example is using the estimate ordetermined value of the resonant frequency f0 to control the referencesignal RS (L) so that the LRA 20 is driven very efficiently in terms ofpower consumption (e.g. at resonance) by the drive output signal dos.

Of course, some of this defining/updating/controlling may be based onpre-set values or input control signals (e.g. received from anothersystem or a user). The present disclosure will be understoodaccordingly. For example, values for the resonant frequency f0 and/orquality factor Q of the LRA 20 may be preset or provided from anexternal system via a control signal.

Merely as examples, the relatively high sample (update) rates indicatedwith the suffix (H) could be at 768 kHz (768000 samples per second) andthe relatively low sample (update) rates indicated with the suffix (L)could be at 48 kHz (48000 samples per second). For example, the signalsRS (H), AS (H), IMON (H), VMON (H) and CS (H) could be 768 kHz digitalsignals, whereas the signals RS (L), IMON (L) and VMON (L) could be 48kHz digital signals. The signal CONFIG (L) might be a 48 kHz signal, ormight have an even lower sample rate (e.g. in the range 1 kHz to 48 kHz,such as 3 kHz). Other sample rates (cf. audio signals) for therelatively low sample (update) rates indicated with the suffix (L) couldbe 44.1 kHz, 88.2 kHz, 96 kHz and 192 kHz (e.g. values within an examplerange of 10 kHz to 200 kHz). These are of course only examples.

Thus, for example, the second control loop (and the fourth control loop)could be 16 times (e.g. between 4 and 100 times) faster than the firstcontrol loop and 16 or 256 times (between 4 and 1000 times) faster thanthe third control loop. These are of course only examples.

As above, the reference signal RS (L) may be used to express hapticsignals, which may have a bandwidth up to 500 Hz or even up to 1 kHz.Note, for use in audio applications, the generally accepted 20 Hz to 20kHz typical range of human hearing—such signals could be expressed by asuitable reference signal RS (L), e.g. with a 44.1 kHz, 48 kHz, 88.2kHz, 96 kHz or 192 kHz sample rate. Again, these values are examples.

Incidentally, the adder 560 and (optional) clipper 570 could beconsidered part of the driver 60, so that the driver 60 receives thereference signal RS (H) and the adjustment signal AS (H) and controlsthe drive output signal dos based on those received signals.

FIG. 6 is a schematic diagram of an AS determiner 550A, as a detailedexample implementation of the AS determiner 550. Thus, in line with theAS determiner 550 of FIG. 5 , the AS determiner 550A is configured togenerate the adjustment signal AS (H) at its output node 602 based onthe current-monitoring signal IMON (H) received at its input node 604.The adjustment signal AS (H) may be considered animpedance-implementation signal.

In overview, the AS determiner 550A comprises a first low-pass filtersection 606, a high-pass filter section 608, a calculation section 610and a second low-pass filter section 612 connected in series between theinput node 604 and the output node 602.

The first low-pass filter section 606 comprises a pair of parallelpaths, one of whose outputs can be selected by a selector based on anenable (selection) signal low1En. One of those paths comprises alow-pass filter, so that the enable signal low1En effectively determineswhether or not the output signal of the first low-pass filter section606 has been subject to low-pass filtering in that section 606.

Similarly, the high-pass filter section 608 comprises a pair of parallelpaths, one of whose outputs can be selected by a selector based on anenable (selection) signal highEn. One of those paths comprises ahigh-pass filter, so that the enable signal highEn effectivelydetermines whether or not the output signal of the high-pass filtersection 608 has been subject to high-pass filtering in that section 608.

Similarly, the second low-pass filter section 612 comprises a pair ofparallel paths, one of whose outputs can be selected by a selector basedon an enable (selection) signal low2En. One of those paths comprises alow-pass filter, so that the enable signal low2En effectively determineswhether or not the output signal of the second low-pass filter section612 has been subject to low-pass filtering in that section 612.

Thus, the high and low-pass filtering may be considered optional (and assuch need not be provided), and may be employed differently in differentapplications.

The calculation section 610 comprises a parallel RLC section 620connected in parallel with a series RLC section 630. The parallel RLCsection 620 comprises a calculation block 622 which operates on thecurrent-monitoring signal IMON (H) to implement the parallel connectionof the parallel resistance par_r, parallel capacitance par_c andparallel inductance par_l of the target output impedance 30, based onparameter or configuration values a0, a1, a2, b0, b1, b2 as indicated.The series RLC section 630 comprises calculation blocks 632, 634 and 636connected together in parallel, which operate on the current-monitoringsignal IMON (H) to implement the series resistance ser_r, seriescapacitance ser_c, and series inductance ser_l, respectively, of thetarget output impedance 30, based on corresponding parameter orconfiguration values ser_r, ser_c and ser_l as indicated.

The outputs of each of the calculation blocks 622, 632, 634, 636 passvia a corresponding AND block along with corresponding enable signalsparEn, rEn, lEn and cEn, respectively, to an adder/subtractor 640 whoseoutput is passed on to the second low-pass filter section 612. In thisway, the contribution of the calculation blocks 622, 632, 634, 636 canbe selectively included or removed from the signal received by thelow-pass filter section 612 by virtue of the respective enable signalsparEn, rEn, lEn and cEn. This, along with control of the parameter orconfiguration values as mentioned above has the effect of being able toconfigure the target output impedance 30 e.g. to take the form of any ofthe target output impedances 30A to 30E (see FIGS. 3A to 3E).

The outputs of the calculation blocks 622, 632, 634, 636 may be referredto as adjustment sub-signals each representative of a corresponding partof the target equivalent circuit representative of the target outputimpedance 30. The adjustment sub-signals may thus be combined to arriveat the adjustment signal AS (H). The calculation blocks 622, 632, 634,636 may perform calculations or access look-up tables, for example.

FIG. 7 is a schematic diagram of an example implementation 50B of thefunction block 50 of FIG. 4 for use in a current-drive mode ofoperation. In this context, it is understood that the operationdescribed in connection with FIGS. 4 to 6 corresponds to animpedance-drive mode of operation. The signals CS (H) and IMON (H) arecarried forwards here for consistency with FIGS. 5 and 6 .

In the current-drive mode of operation, the function block 50 isconfigured to function in line with the example implementation 50B, andin particular to generate the control signal CS (H) as the result ofsubtracting (at a subtractor 702) the current-monitoring signal IMON (H)acting as a feedback signal from the reference signal RS (H). Thisnegative feedback operation enables the current flowing through the LRA20 to be controlled based on the reference signal RS (H).

It will be understood that the function block 50 of FIG. 4 may beconfigured to operate selectively in the impedance-drive mode ofoperation (in line with FIGS. 5 and 6 ) or the current-drive mode ofoperation (in line with FIG. 7 ), for example based on a mode-selectionsignal (which may be supplied by the controller 70, e.g. as part of theCONFIG signal).

FIG. 8 presents a series of graphs A to D (labelled clockwise startingfrom the top left) useful for understanding the benefits andcapabilities of the driver circuitry 40A, 40B disclosed herein.

These Bode plots compare the position, velocity, acceleration and powertransfer functions in various modes for driving an example typical LRA20 with resonant frequency f0=50 Hz, and quality factor Q=3, using thedriver circuitry 40A, 40B.

Graph A considers driving the LRA 20 without simulating the presence ofthe target output impedance 30 or by simulating the presence of thetarget output impedance 30 when configured to have zero impedance. Thisis equivalent to driving the LRA 20 in line with FIG. 1 . This form ofdriving provides relatively poor mechanical control of the LRA 20.

Graph B considers driving the LRA 20 (in impedance-drive mode), with thesimulated target output impedance 30 being configured as in the variant30A of FIG. 3A, i.e. as a negative impedance (negative resistance). Thisform of driving exhibits a constant velocity transfer function from 20Hz to 200 Hz.

Graph C considers driving the LRA 20 (in impedance-drive mode), with thesimulated target output impedance 30 being configured as in the variant30E of FIG. 3E, i.e. as a positive impedance (positive resistance) wherethe impedance (resistance) value is much larger (e.g. 10 x) than that ofthe LRA 20. This form of driving exhibits a constant position transferfunction below resonance (from DC to 20 Hz) and a constant accelerationtransfer function above resonance (from 200 Hz to 1 KHz), but at thecost of high impedance.

Graph D considers driving the LRA 20, but using the current-drive modeof operation in line with FIG. 7 (i.e. without simulating the presenceof the target output impedance 30). This form of driving exhibits aconstant position transfer function below resonance (from DC to 20 Hz)and a constant acceleration transfer function above resonance (from 200Hz to 1 KHz) similar to Graph C, but without needing the high impedance.

The acceleration waveform of typical LRAs 20 with low Q can becontrolled accurately over the full haptic sensitivity range (DC-500 Hz)using negative impedance (Graph B) around resonance (20-200 Hz) andcurrent drive (Graph D) above resonance (>200 Hz).

A so-called “poor man's” current drive can be realized by configuringthe negative impedance circuit to have large positive impedance (GraphC).

At this juncture it is noted that the driver circuitry arrangements havebeen described so far based on voltage source driving of the LRA 20(electromechanical load), i.e. with the drive output signal dos being avoltage signal vload (and the reference signal RS being configured forvoltage driving). This drive output signal dos induces a load currentiload to be drawn by (or to flow through) the LRA 20. With this form ofdriving in mind, the load current iload is monitored (e.g. using signalIMON) and used to determine a voltage which would be induced across thetarget output impedance 30 so as to generate the adjustment signal AS.The driver 60 is configured to generate the drive output signal dosbased on the reference signal RS and the adjustment signal AS to causethe drive output signal dos to behave as if an output impedance of thedriver circuitry has been adjusted to comprise the target outputimpedance. FIG. 9A is a schematic diagram of the modified drivercircuitry 40 of FIG. 2 as a reminder of this voltage source control,with the LRA 20 shown connected thereto for completeness.

However, driver circuitry arrangements are also envisaged based oncurrent source driving of the LRA 20. It will be appreciated that (withthe principles of source transformation in mind) it would be possible toactively control the load current iload (rather than the load voltagevload) to drive the LRA 20 by current source control in an equivalentway to the driving by voltage source control.

FIG. 9B is a schematic diagram of modified driver circuitry 40C which isequivalent to the modified driver circuitry 40 of FIG. 9A, but whichuses current source control. Like elements are denoted by like referencesigns. The voltage source 10 of FIG. 9B has been replaced with a currentsource 10C which provides a current reference signal Iref. Further,instead of providing the target output impedance (virtual impedance) 30in series with the voltage source 10 as in FIG. 9A, it is provided in areconfigured format in FIG. 9B in which the series components (ser_r,ser_l, ser_c) are in series with the current source 10C, and theparallel components (par_r, par_l, par_c) are in parallel with thecurrent source 10C. The LRA 20 (separate from the modified drivercircuitry 40 and 40C) is connected in the same way in both cases.

Therefore (with FIG. 9B in mind) the driver circuitry 40A and 40B ofFIGS. 4 to 6 could be converted into equivalent driver circuitry basedon current source driving of the LRA in line with driver circuitry 40C,i.e. with the drive output signal dos being a current signal iload (andthe reference signal RS being configured for current driving). Thisdrive output signal dos induces a load voltage vload across the LRA 20.With this form of driving in mind, the load voltage vload may bemonitored (e.g. using signal VMON) and used to determine a current whichwould be induced to flow through the target output impedance so as togenerate the adjustment signal AS (i.e. so that the adjustment signal ASrepresents a current rather than a voltage). The driver 60 (a currentamplifier, in particular a high-speed or wide-bandwidth currentamplifier) may then be configured to generate the drive output (current)signal dos based on the (current based) reference signal RS and the(current based) adjustment signal AS to cause the drive output signaldos to behave as if an output impedance of the driver circuitry has beenadjusted to comprise the target output impedance.

Thus, the description of FIGS. 4 to 6 may be considered accordingly andunderstood to apply mutatis mutandis to equivalent current source drivenarrangements. That is, the driver circuitry 40A and 40B will beunderstood to have current source driven equivalents to which thetechniques described herein apply analogously.

For example, in the voltage source driven arrangements described earlierthe AS determiner 550 determines (e.g. by calculation or using a look-uptable) a voltage which would be induced across the target outputimpedance 30 if the current flowing through the LRA 20 were to flowthrough the target output impedance 30. In equivalent current sourcedriven arrangements, the AS determiner 550 determines (e.g. bycalculation or using a look-up table) a current which would be drawn bythe target output impedance in view of the voltage across the LRA 20.Similarly, the adjustment sub-signals were described in terms ofvoltages in the voltage source driven arrangements but would be currentsin equivalent current source driven arrangements.

As another example, in the voltage source driven arrangements describedearlier the second control loop (implementing the target outputimpedance) uses the current monitor signal IMON to adjust a voltagesignal and the fourth control loop (aiming to achieve linear operationof the driver 60) uses the voltage monitor signal VMON to adjust avoltage signal. In equivalent current source driven arrangements, thesecond control loop (implementing the target output impedance) uses thevoltage monitor signal VMON to adjust a current signal and the fourthcontrol loop (aiming to achieve linear operation of the driver 60) usesthe current monitor signal IMON to adjust a current signal. Indeed, oneskilled in the art will appreciate that the logic used for the secondand fourth control loops in the voltage source driven arrangements couldeffectively be swapped around (with suitable changes to the referencesignal RS) to lead to current source driven arrangements.

FIG. 10 is a schematic diagram of host device 1000 which comprises thedriver circuitry 40A or 40B (assuming the voltage source controlversions as explained in connection with FIGS. 4 and 5 , or currentsource control versions as introduced in connection with FIG. 9B) andthe LRA 20, with the driver circuitry 40A or 40B connected to drive theLRA 20. The host device 1000 may of course comprise other components(not shown) to control or operate alongside the driver circuitry, suchas an applications processor.

The skilled person will recognise that some aspects of the abovedescribed apparatus (circuitry) and methods may be embodied as processorcontrol code, for example on a non-volatile carrier medium such as adisk, CD- or DVD-ROM, programmed memory such as read only memory(Firmware), or on a data carrier such as an optical or electrical signalcarrier.

For some applications, such aspects will be implemented on a DSP(Digital Signal Processor), ASIC (Application Specific IntegratedCircuit) or FPGA (Field Programmable Gate Array). Thus the code maycomprise conventional program code or microcode or, for example, codefor setting up or controlling an ASIC or FPGA. The code may alsocomprise code for dynamically configuring re-configurable apparatus suchas re-programmable logic gate arrays. Similarly, the code may comprisecode for a hardware description language such as Verilog™ or VHDL. Asthe skilled person will appreciate, the code may be distributed betweena plurality of coupled components in communication with one another.Where appropriate, such aspects may also be implemented using coderunning on a field-(re)programmable analogue array or similar device inorder to configure analogue hardware.

Some embodiments of the present invention may be arranged as part of ahaptic circuit, for instance a haptic circuit which may be provided in ahost device 1000 as discussed above. A circuit or circuitry according toan embodiment of the present invention (such as driver circuitry 40A or40B) may be implemented (at least in part) as an integrated circuit(IC), for example on an IC chip. One or more input or output transducers(such as LRA 20) may be connected to the integrated circuit in use.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in the claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope.

The present disclosure extends to the following set A of statements:

A1. Driver circuitry for driving an electromechanical load with a driveoutput signal based on a digital reference signal at a first samplerate, the drive output signal inducing a first electrical quantity atthe electromechanical load, the driver circuitry comprising:

-   -   a function block configured, based on said first electrical        quantity, to digitally determine at a second sample rate higher        than the first sample rate an adjustment signal indicative of a        second electrical quantity which would be induced at a target        output impedance of the driver circuitry due to said first        electrical quantity; and    -   a driver configured to generate the drive output signal based on        the reference signal and the adjustment signal to cause the        drive output signal to behave as if an output impedance of the        driver circuitry has been adjusted to comprise the target output        impedance,    -   wherein the first electrical quantity is a current and the        second electrical quantity is a voltage, or vice versa.

A2. The driver circuitry according to statement A1, wherein:

-   -   the drive output signal is a voltage signal, the first        electrical quantity is a current drawn by the electromechanical        load and the second electrical quantity is a voltage across the        target output impedance; or    -   the drive output signal is a current signal, the first        electrical quantity is a voltage across the electromechanical        load and the second electrical quantity is a current drawn by        the target output impedance.

A3. The driver circuitry according to statement A1 or A2, wherein thefunction block is configured to digitally determine the adjustmentsignal based on said first electrical quantity and a definition of saidtarget output impedance.

A4. The driver circuitry according to statement A3, wherein thedefinition comprises one or more configuration values.

A5. The driver circuitry according to statement A4, comprising storagefor storing the one or more configuration values, wherein an impedancevalue of the target output impedance is maintained when the one or moreconfiguration values stored in the storage are maintained.

A6. The driver circuitry according to any of statements A4 or A5,wherein:

-   -   a target equivalent circuit representative of the target output        impedance comprises one or more impedance components and a        circuit structure for connecting the one or more impedance        components together; and    -   the one or more configuration values define at least one said        impedance component and/or said circuit structure.

A7. The driver circuitry according to statement A6, wherein:

-   -   the target equivalent circuit comprises a plurality of impedance        components connected together; and    -   the function block is configured, based on said first electrical        quantity and the one or more configuration values, to:    -   determine a plurality of adjustment sub-signals each        representative of a corresponding part of the target equivalent        circuit and indicative of a portion of the second electrical        quantity which would be induced at the corresponding part of the        target equivalent circuit if said second electrical quantity        were induced at the target equivalent circuit; and    -   determine the adjustment signal by combining the plurality of        adjustment sub-signals,    -   and optionally wherein:    -   the portion of the second electrical quantity is a voltage        across the corresponding part of the target equivalent circuit        if the second electrical quantity is the voltage across the        target output impedance; and    -   the portion of the second electrical quantity is a current drawn        by the corresponding part of the target equivalent circuit if        the second electrical quantity is the current drawn by the        target output impedance.

A8. The driver circuitry according to statement A6 or A7, wherein:

-   -   the one or more configuration values define the target        equivalent circuit to comprise at least one of a series        resistor, a series capacitor, a series inductor and a parallel        network of impedances, the parallel network of impedances        comprising at least two of a parallel resistor, a parallel        capacitor and a parallel inductor connected together in        parallel, each of those resistors, capacitors and inductors        being a said impedance component,    -   optionally wherein those of the series resistor, the series        capacitor, the series inductor and the parallel network of        impedances present in the target equivalent circuit are        connected in series.

A9. The driver circuitry according to statement A8, wherein the one ormore configuration values define the target equivalent circuit tocomprise, optionally only:

-   -   the series resistor, wherein the series resistor has a negative        resistance;    -   the series resistor and the series inductor connected together        in series, wherein the series resistor has a negative resistance        and the series inductor has a negative inductance;    -   the series resistor and the series inductor connected together        in series and to the parallel network of impedances, wherein the        series resistor has a negative resistance and the series        inductor has a negative inductance, and wherein the parallel        network of impedances comprises the parallel resistor, the        parallel capacitor and the parallel inductor connected together        in parallel;    -   the series resistor and the series capacitor connected together        in series, wherein the series resistor has a negative resistance        and the series capacitor has a positive capacitance; or    -   the series resistor, wherein the series resistor has a positive        resistance, and wherein the positive resistance is substantially        larger than a resistance of the electromechanical load, or than        a resistance of a resistor in an electromechanical-load        equivalent circuit representing a mechanical impedance of the        electromechanical load.

A10. The driver circuitry according to any of the preceding Astatements, comprising a controller configured:

-   -   to generate the reference signal based on a drive input signal        and based on a current drawn by the electromechanical load        and/or a voltage across the electromechanical load; and/or    -   to control, based on said current drawn by the electromechanical        load and/or said voltage across the electromechanical load, a        definition of the target output impedance to cause a performance        such as a mechanical performance of the electromechanical load        to meet a performance target; and/or    -   to control, based on said current drawn by the electromechanical        load and/or said voltage across the electromechanical load, a        definition of the target output impedance to cause the target        output impedance to cancel an impedance of at least one        electrical component of the electromechanical load, optionally a        coil such as a voice coil; and/or    -   to control a definition of the target output impedance based on        an impedance control signal to cause a performance of the driver        circuitry to vary with the impedance control signal.

A11. The driver circuitry according to any of the preceding Astatements, wherein the driver is configured to generate the driveoutput signal so that the drive output signal has a predefinedrelationship with a summation of the adjustment signal and the referencesignal.

A12. The driver circuitry according to any of the preceding Astatements, wherein:

-   -   the function block is configured to generate a control signal        having a predefined relationship with a summation of the        adjustment signal and the reference signal; and    -   the driver is configured to generate the drive output signal so        that the drive output signal has a predefined relationship with        the control signal.

A13. The driver circuitry according to statement A12, wherein:

-   -   the driver circuitry is selectively operable in an        impedance-drive mode or a current-drive mode;    -   the control signal is generated based on the reference signal        and the adjustment signal so that the drive output signal        behaves as if the output impedance of the driver circuitry has        been adjusted to comprise the target output impedance when the        driver circuitry is in the impedance-drive mode; and    -   in the current-drive mode, the function block is configured to        generate the control signal as a function of a current-control        reference signal and a current drawn by the electromechanical        load, and to adjust the control signal based on said current        drawn by the electromechanical load so that said current drawn        by the electromechanical load has a predefined relationship with        the current-control reference signal.

A14. The driver circuitry according to statement A12 or A13, wherein:

-   -   at least one of the control signal and the adjustment signal is        a digital signal;    -   the control signal and the adjustment signal are digital        signals, and the function block is a digital function block;        and/or    -   the drive output signal is an analogue signal.

A15. The driver circuitry according to any of statements A12 to A14,wherein:

-   -   the control signal is a digital signal; and    -   the driver comprises a digital-to-analogue converter and an        analogue amplifier connected together to convert the control        signal into an analogue signal and then amplify that analogue        signal to form the drive output signal.

A16. The driver circuitry according to any of the preceding Astatements, comprising a monitoring unit configured to generate acurrent monitoring signal indicative of a current drawn by theelectromechanical load and/or a voltage monitoring signal indicative ofa voltage across the electromechanical load, wherein the function blockis configured to digitally determine the adjustment signal based on thecurrent monitoring signal and/or the voltage monitoring signal.

A17. The driver circuitry according to any of the preceding Astatements, wherein:

-   -   the reference signal is indicative of an intended mechanical        performance of the electromechanical load; and/or    -   the behaviour of the drive output signal as if the output        impedance of the driver circuitry has been adjusted to comprise        the target output impedance is relative to an expected behaviour        of an expected drive output signal expected to be generated by        the driver based on the reference signal without the adjustment        signal; and/or    -   the driver circuitry comprises one or more analogue impedance        components connected to contribute to the output impedance of        the driver circuitry; and/or    -   the target output impedance is configured to cancel an impedance        of at least one electrical component of the electromechanical        load, optionally a coil such as a voice coil; and/or    -   said electromechanical load is an electromechanical device such        as an actuator; and/or    -   said electromechanical load is a resonant electromechanical load        such as a linear resonant actuator, a speaker or a microspeaker.

A18. The driver circuitry according to any of the preceding Astatements, wherein:

-   -   the driver forms part of a first control loop operable to        control the drive output signal based on the reference signal;    -   the driver and the function block form part of a second control        loop operable to control the drive output signal based on a        current drawn by the electromechanical load and/or a voltage        across the electromechanical load; and    -   the second control loop is configured to have a lower latency        than the first control loop.

A19. The driver circuitry according to statement A18, wherein at leastpart of the first control loop and at least part of the second controlloop are implemented as digital circuitry, and wherein the latencies ofthe first and second control loops are defined by sample rates ofrespective digital signals of the first and second control loops.

A20. The driver circuitry according to any of the preceding Astatements, comprising an analogue impedance configured to form part ofthe output impedance of the driver circuitry,

-   -   optionally wherein the analogue impedance is a controllable        analogue impedance and the function block is configured to        control the controllable analogue impedance to adjust the output        impedance of the driver circuitry.

A21. The driver circuitry according to statement A20, configured tocontrol a definition of the target output impedance and/or an impedanceof the analogue impedance to control the output impedance of the drivercircuitry.

A22. The driver circuitry according to any of the preceding Astatements, implemented as integrated circuitry such as on an IC chip.

A23. An IC chip comprising the driver circuitry according to any of thepreceding A statements.

A24. A control system, comprising:

-   -   the driver circuitry according to any of the preceding A        statements; and    -   the electromechanical load,    -   wherein the electromechanical load is connected to be driven by        said drive output signal.

A25. A haptic system comprising the control system of statement A24,wherein the electromechanical load is a linear resonant actuator coupledto a physical structure or surface of the system to produce a hapticeffect for a user.

A26. A host device, such as portable electrical or electronic device,comprising the driver circuitry according to any of statements A1 toA22, or the IC chip of statement A23, or the control system of statementA24 or the haptic system of statement A25.

A27. A method carried out by driver circuitry to drive anelectromechanical load with a drive output signal based on a digitalreference signal, the drive output signal inducing a first electricalquantity at the electromechanical load, the method comprising:

-   -   based on said first electrical quantity, digitally determining        at a second sample rate higher than the first sample rate an        adjustment signal indicative of a second electrical quantity        which would be induced at a target output impedance of the        driver circuitry due to said first electrical quantity; and    -   generating the drive output signal based on the reference signal        and the adjustment signal to cause the drive output signal to        behave as if an output impedance of the driver circuitry has        been adjusted to comprise the target output impedance,    -   wherein the first electrical quantity is a current and the        second electrical quantity is a voltage, or vice versa.

The present disclosure extends to the following set B of statements:

B1. Driver circuitry for driving an electromechanical load with a driveoutput signal, the driver circuitry comprising:

-   -   a first control loop operable to control the drive output signal        based on a drive input signal; and    -   a second control loop operable to control the drive output        signal based on a current flowing through and/or a voltage        induced across the electromechanical load,    -   wherein the second control loop is configured to have a lower        latency than the first control loop.

B2. The driver circuitry according to statement B1, wherein the secondcontrol loop is configured to control the drive output signal tocompensate for an impedance of the electromechanical load.

B3. The driver circuitry according to statement B1 or B2, wherein thesecond control loop is configured to control the drive output signal sothat it behaves as if an output impedance of the driver circuitry hasbeen adjusted to comprise a target output impedance.

B4. The driver circuitry according to statement B3, wherein the driveoutput signal is a voltage signal and the second control loop isconfigured to perform its control of the drive output signal based on avoltage signal which would be induced across the target output impedanceby said current,

-   -   optionally wherein the second control loop is configured to        determine, based on said current, an adjustment signal        indicative of said voltage signal, and control the drive output        signal based on the adjustment signal.

B5. The driver circuitry according to statement B3, wherein the driveoutput signal is a current signal and the second control loop isconfigured to perform its control of the drive output signal based on acurrent signal of a current which would be induced to flow through thetarget output impedance by said voltage,

-   -   optionally wherein the second control loop is configured to        determine, based on said voltage, an adjustment signal        indicative of said current signal, and control the drive output        signal based on the adjustment signal.

B6. The driver circuitry according to any of the preceding B statements,comprising a third control loop operable to:

-   -   determine, based on said current and/or said voltage, one or        more configuration values for defining the target output        impedance; and    -   provide the determined configuration values to the second        control loop to define the target output impedance.

B7. The driver circuitry according to statement B6, wherein the latencyof the second control loop is lower than a latency of the third controlloop.

B8. The driver circuitry according to any of the preceding B statements,wherein the first control loop is configured for feedback control of theelectromechanical load based on said current and/or said voltage.

B9. The driver circuitry according to any of the preceding B statements,wherein the first control loop is configured for feedforward control ofthe electromechanical load.

B10. The driver circuitry according to any of the preceding Bstatements, wherein the second control loop is a feedback control loop,said current and/or said voltage being a feedback signal in the secondcontrol loop.

B11. The driver circuitry according to any of the preceding Bstatements, wherein at least part of the first control loop and at leastpart of the second control loop are implemented as digital circuitry,and wherein the latencies of the first and second control loops aredefined by sample rates of respective digital signals of the first andsecond control loops.

B12. The driver circuitry according to any of the preceding Bstatements, comprising:

-   -   a monitor unit configured to monitor said current and/or said        voltage and generate a monitor signal indicative of said current        and/or said voltage;    -   a controller operable to generate a reference signal based on        said drive input signal and said monitor signal;    -   a function block operable to generate an adjustment signal based        on said monitor signal; and    -   a driver operable to generate said drive output signal based on        said reference signal and said adjustment signal.

B13. The driver circuitry according to statement B12, wherein:

-   -   said first control loop comprises a first signal path which        extends from said monitor unit to said driver via said        controller, the first signal path carrying said monitor signal        and said reference signal;    -   said second control loop comprises a second signal path which        extends from said monitor unit to said driver via said function        block, the second signal path carrying said monitor signal and        said adjustment signal;    -   at least one signal carried by said first control loop and one        or more signals carried by said second control loop are digital        signals; and    -   the one or more digital signals carried by said second control        loop have a higher sample rate than the at least one digital        signal carried by said first control loop.

B14. The driver circuitry according to statement B13, wherein:

-   -   at least one signal carried by said first control loop between        said monitor unit and said controller and at least one signal        carried by said first control loop between said controller and        said driver are digital signals; and    -   the one or more digital signals carried by said second control        loop have a higher sample rate than the at least one digital        signal carried by said first control loop between said monitor        unit and said controller and/or the at least one signal carried        by said first control loop between said controller and said        driver.

B15. The driver circuitry according to any of statements B12 to B14,wherein:

-   -   the function block is operable to generate a control signal        based on said adjustment signal and said reference signal; and    -   the driver is operable to generate said drive output signal        based on said control signal.

B16. The driver circuitry according to any of the preceding Bstatements, wherein:

-   -   the reference signal and/or the drive input signal is indicative        of an intended mechanical performance of the electromechanical        load; and/or    -   said electromechanical load is an electromechanical device such        as an actuator; and/or    -   said electromechanical load is a resonant electromechanical load        such as a linear resonant actuator, a speaker or a microspeaker.

B17. The driver circuitry according to any of the preceding Bstatements, implemented as integrated circuitry such as on an IC chip.

B18. An IC chip comprising the driver circuitry according to any of thepreceding B statements.

B19. A control system, comprising:

-   -   the driver circuitry according to any of the preceding B        statements; and    -   the electromechanical load,    -   wherein the electromechanical load is connected to be driven by        said drive output signal.

B20. A haptic system comprising the control system of statement B19,wherein the electromechanical load is a linear resonant actuator coupledto a physical structure or surface of the system to produce a hapticeffect for a user.

B21. A host device, such as portable electrical or electronic device,comprising the driver circuitry according to any of statements B1 toB17, or the IC chip of statement B18, or the control system of statementB19 or the haptic system of statement B20.

B22. A method of driving an electromechanical load with a drive outputsignal, the method comprising:

-   -   with a first control loop, controlling the drive output signal        based on a drive input signal; and    -   with a second control loop, controlling the drive output signal        based on a current flowing through and/or a voltage induced        across the electromechanical load,    -   wherein the second control loop is configured to have a lower        latency than the first control loop.

The present disclosure extends to the following set C of statements:

C1. Driver circuitry for driving an electromechanical load with a driveoutput signal based on a reference signal, the drive output signalinducing a first electrical quantity at the electromechanical load, thedriver circuitry comprising:

-   -   a function block configured, based on said first electrical        quantity, to digitally determine an adjustment signal indicative        of a second electrical quantity which would be induced at a        target output impedance of the driver circuitry due to said        first electrical quantity; and    -   a driver configured to generate the drive output signal based on        the reference signal and the adjustment signal to cause the        drive output signal to behave as if an output impedance of the        driver circuitry has been adjusted to comprise the target output        impedance,    -   wherein:    -   the drive output signal is a voltage signal, the first        electrical quantity is a current drawn by the electromechanical        load and the second electrical quantity is a voltage across the        target output impedance; or    -   the drive output signal is a current signal, the first        electrical quantity is a voltage across the electromechanical        load and the second electrical quantity is a current drawn by        the target output impedance.

C2. Driver circuitry for driving an electromechanical load with a driveoutput signal based on a reference signal, the drive output signal beinga voltage signal and causing a current to be drawn by theelectromechanical load, the driver circuitry comprising:

-   -   a function block configured, based on said current, to digitally        determine an adjustment signal indicative of a voltage signal        which would be induced across a target output impedance of the        driver circuitry by said current; and    -   a driver configured to generate the drive output signal based on        the reference signal and the adjustment signal to cause the        drive output signal to behave as if an output impedance of the        driver circuitry has been adjusted to comprise the target output        impedance.

C3. Driver circuitry for driving a linear resonant actuator, the drivercircuitry comprising:

-   -   a function block configured to generate a digital control signal        as a function of a digital reference signal, intended for        controlling the linear resonant actuator, and a monitor signal;        and    -   a driver configured to convert the digital control signal into        an analogue drive signal to drive the linear resonant actuator,    -   wherein:    -   the monitor signal is indicative of a current flowing through,        and/or a voltage across, the linear resonant actuator; and    -   the function block is configured, based on the monitor signal,        to control a difference between the digital control signal and        the digital reference signal so that the analogue drive signal        when driving the linear resonant actuator has a target behaviour        in which the analogue drive signal behaves, relative to an        expected analogue drive signal expected to be generated with the        digital control signal being the digital reference signal, as if        the output impedance of the driver circuitry has been adjusted        to comprise a target output impedance.

C4. Driver circuitry for driving an electromechanical load with a driveoutput signal based on a reference signal, the driver circuitryconfigured to generate the drive output signal based on a digitaloperation dependent on the reference signal and an electrical quantityinduced at the electromechanical load to cause the drive output signalto behave as if an output impedance of the driver circuitry has beenadjusted to comprise a target output impedance.

C5. Driver circuitry for driving an electromechanical load with a driveoutput signal based on a reference signal, the drive output signalinducing a first electrical quantity at the electromechanical load, thedriver circuitry comprising:

-   -   a function block configured, based on said first electrical        quantity, to digitally determine an adjustment signal indicative        of a second electrical quantity which would be induced at a        target output impedance of the driver circuitry due to said        first electrical quantity; and    -   a driver configured to generate the drive output signal based on        the reference signal and the adjustment signal to cause the        drive output signal to behave as if an output impedance of the        driver circuitry has been adjusted to comprise the target output        impedance.

C6. Driver circuitry for driving an electromechanical load with a driveoutput signal based on a reference signal, the driver circuitryconfigured to digitally control the drive output signal based on thereference signal to cause the drive output signal to behave as if anoutput impedance of the driver circuitry has been adjusted to comprise adefined or predetermined target output impedance.

C7. Driver circuitry for driving an electromechanical load with a driveoutput signal based on a reference signal, the driver circuitryconfigured to digitally control the drive output signal based on thereference signal and an electrical quantity at the electromechanicalload to cause the drive output signal to behave as if an outputimpedance of the driver circuitry has been adjusted to comprise adefined or predetermined target output impedance.

The invention claimed is:
 1. Driver circuitry for driving anelectromechanical load with a drive output signal, the driver circuitrycomprising: a first control loop operable to control the drive outputsignal based on a drive input signal; and a second control loop operableto control the drive output signal based on a current flowing throughand/or a voltage induced across the electromechanical load, wherein thesecond control loop is configured to have a lower latency than the firstcontrol loop, and to control the drive output signal to compensate foran impedance of the electromechanical load.
 2. The driver circuitryaccording to claim 1, wherein the second control loop is configured tocontrol the drive output signal so that it behaves as if an outputimpedance of the driver circuitry has been adjusted to comprise a targetoutput impedance.
 3. The driver circuitry according to claim 2, whereinthe drive output signal is a voltage signal and the second control loopis configured to perform its control of the drive output signal based ona voltage signal which would be induced across the target outputimpedance by said current, optionally wherein the second control loop isconfigured to determine, based on said current, an adjustment signalindicative of said voltage signal, and control the drive output signalbased on the adjustment signal.
 4. The driver circuitry according toclaim 2, wherein the drive output signal is a current signal and thesecond control loop is configured to perform its control of the driveoutput signal based on a current signal of a current which would beinduced to flow through the target output impedance by said voltage,optionally wherein the second control loop is configured to determine,based on said voltage, an adjustment signal indicative of said currentsignal, and control the drive output signal based on the adjustmentsignal.
 5. The driver circuitry according to claim 2, comprising a thirdcontrol loop operable to: determine, based on said current and/or saidvoltage, one or more configuration values for defining the target outputimpedance; and provide the determined configuration values to the secondcontrol loop to define the target output impedance.
 6. The drivercircuitry according to claim 5, wherein the latency of the secondcontrol loop is lower than a latency of the third control loop.
 7. Thedriver circuitry according to claim 1, wherein the first control loop isconfigured for feedback control of the electromechanical load based onsaid current and/or said voltage.
 8. The driver circuitry according toclaim 1, wherein the first control loop is configured for feedforwardcontrol of the electromechanical load.
 9. The driver circuitry accordingto claim 1, wherein the second control loop is a feedback control loop,said current and/or said voltage being a feedback signal in the secondcontrol loop.
 10. The driver circuitry according to claim 1, wherein atleast part of the first control loop and at least part of the secondcontrol loop are implemented as digital circuitry, and wherein thelatencies of the first and second control loops are defined by samplerates of respective digital signals of the first and second controlloops.
 11. The driver circuitry according to claim 1, comprising: amonitor unit configured to monitor said current and/or said voltage andgenerate a monitor signal indicative of said current and/or saidvoltage; a controller operable to generate a reference signal based onsaid drive input signal and said monitor signal; a function blockoperable to generate an adjustment signal based on said monitor signal;and a driver operable to generate said drive output signal based on saidreference signal and said adjustment signal.
 12. The driver circuitryaccording to claim 11, wherein: said first control loop comprises afirst signal path which extends from said monitor unit to said drivervia said controller, the first signal path carrying said monitor signaland said reference signal; said second control loop comprises a secondsignal path which extends from said monitor unit to said driver via saidfunction block, the second signal path carrying said monitor signal andsaid adjustment signal; at least one signal carried by said firstcontrol loop and one or more signals carried by said second control loopare digital signals; and the one or more digital signals carried by saidsecond control loop have a higher sample rate than the at least onedigital signal carried by said first control loop.
 13. The drivercircuitry according to claim 12, wherein: at least one signal carried bysaid first control loop between said monitor unit and said controllerand at least one signal carried by said first control loop between saidcontroller and said driver are digital signals; and the one or moredigital signals carried by said second control loop have a higher samplerate than the at least one digital signal carried by said first controlloop between said monitor unit and said controller and/or the at leastone signal carried by said first control loop between said controllerand said driver.
 14. The driver circuitry according to claim 11,wherein: the function block is operable to generate a control signalbased on said adjustment signal and said reference signal; and thedriver is operable to generate said drive output signal based on saidcontrol signal.
 15. The driver circuitry according to claim 11, wherein:the reference signal and/or the drive input signal is indicative of anintended mechanical performance of the electromechanical load; and/orsaid electromechanical load is an electromechanical device such as anactuator; and/or said electromechanical load is a resonantelectromechanical load such as a linear resonant actuator, a speaker ora microspeaker.
 16. An IC chip comprising the driver circuitry accordingto claim
 1. 17. A control system, comprising: the driver circuitryaccording to claim 1; and the electromechanical load, wherein theelectromechanical load is connected to be driven by said drive outputsignal.
 18. A haptic system comprising the control system of claim 17,wherein the electromechanical load is a linear resonant actuator coupledto a physical structure or surface of the system to produce a hapticeffect for a user.
 19. A host device, such as portable electrical orelectronic device, comprising the driver circuitry according to claim 1.20. Driver circuitry for driving an electromechanical load with a driveoutput signal, the driver circuitry comprising: a first control loopoperable to control the drive output signal based on a drive inputsignal; and a second control loop operable to control the drive outputsignal based on a current flowing through and/or a voltage inducedacross the electromechanical load, wherein: the second control loop isconfigured to have a lower latency than the first control loop; at leastpart of the first control loop and at least part of the second controlloop are implemented as digital circuitry; and the latencies of thefirst and second control loops are defined by sample rates of respectivedigital signals of the first and second control loops.